That doesn't make sense. You have the P5 ECU and you're too rich?! You should be too lean instead.. it must be the Walbro pump dumping in the extra fuel. 808mp5 had the same setup as you and his AFRs were about the same up to 4 psi, then would go up into 13-14 range after that. Is that what happens to you or do you see a constant 10 range no matter how much you're boosting?
Either way, I would suggest that you invest in some type of fuel management.. cuz the P5 ECU is NOT meant for boost. When you hit boost, more fuel is dumped in.. and the P5 ECU is programmed to cut the extra fuel in order to keep the AFR's around 14.7. Then go back to the stock fuel pump, cuz like lil_red mentioned the stock FPR doesn't work well with the Walbro.
The easiest method is the MSP ECU, which plugs right in (only if you have 5-speed, will NOT work with automatic transmissions). Or you can go with the SS AFC, it's relatively cheap and easy to install, and allows you to effectively tune your AFRs to an optimal point. A LOT of members here use it.
Another alternative (one I am using because I can't use the MSP ECU due to my automatic tranny) is adding a Rising Rate FPR after the stock FPR. The BEGi 2025 unit is what I use but you'll also need the Flyin' Miata o2 sensor clamp to trick the P5 ECU into not seeing the extra fuel. It's not a very popular choice anymore, but for stock boost (6 psi) and stock fuel pump/injectors it works perfect. My AFR's are 14.7-15.5 under vacuum, and 11.0 - 12.5 while boosting.
